Field mounted control unit

ABSTRACT

A smart field-mounted control unit, for controlling a process, receives signals and sends a command output over a two-wire circuit which powers the control unit. An input section receives the signal, which can be instructions representative of commands or instructions sets, process variables sensed by external control units or setpoints representative of a desired process state. The instructions are representative of a control requirement of the process and adjust a controlling section in the control unit to generate the command output in conformance with the control requirement. The command output can be a function of the difference between the process setpoint and a process variable, or a function of a linear combination of a process variable and its calculated time integral and time derivative functions. A sensing section in the control unit can sense and scale a process variable for generating the command output as well, The control unit can include a regulator section, controlled by the command output, which regulates application of a mechanical, hydraulic, pneumatic or electromagnetic force applied to the process.

This is a continuation of application Ser. No. 08/002,257 filed on Jan.8, 1993, abandoned as of the date of this application which is acontinuation of application Ser. No. 07/785,168 filed Nov. 13, 1991,abandoned Jan. 8, 1993, which is a continuation of application Ser. No.07/416,180 filed Oct. 2, 1989, abandoned Nov. 13, 1991.

FIELD OF THE INVENTION

The present invention relates to a transmitter communicating over atwo-wire circuit and providing an output representative of a processvariable produced by a process.

BACKGROUND OF THE INVENTION

Transmitters measure process variables representative of a processcontrolled by remote devices and communicate transmitter outputsrepresenting the process variables to controllers over two wirecircuits. The transmitters are typically mounted in a field area wherecurrent and voltage levels are limited to provide intrinsic safety. Thetransmitter output is scaled by user definable parameters such as span,zero and damping. Span and zero adjustments allow the user to referencethe measurement range extremes of the transmitter to specifictransmitter output levels, thereby setting the range of desired outputs.Damping affects the response time of the transmitter to changes in theprocess variable. The scaled transmitter output wire is sent over thetwo wire circuit to the controller.

Controllers, typically located in a control room, combine thetransmitter output with signals representing other process variables togenerate command output signals. Command output signals are typicallysent over a separate pair of wires to remote devices, such as a valve,which control the process according to the command output. In certainapplications, controllers select the most appropriate set ofinstructions for process control from multiple sets of instructions,depending on the process to be controlled and the accuracy required.

In other applications, controllers sense several transmitter outputsrepresenting process variables to determine the command output for theremote device. Typically, a separate transmitter senses each processvariable. The transmitters send a signal representative of the sensedprocess variable to the controller over a two wire circuit, thecontroller determines the command output and controls the remote device,such as a valve position, pump speed, thermostat setting, etc.

One limitation of the prior art arrangement is that the transmitter,remote device and controller, components in the feedback loop, must alloperate continuously for control of the process. Another limitation isthe amount of interconnecting cabling connecting the feedback loopcomponents. The controller is typically far from the process in acontrol room, while the remote device and the transmitter are usually inthe field and physically proximate to each other and the process.Installation and maintenance complexity is another limitation, sinceeach cable may require installation of an intrinsic safety barrierdevice at the interface between the control room and the field devices.In cases where multiple process variables are used by the controller,cabling is required between each transmitter and the controller.Feedback loop reliability is a fourth limitation, since failure of oneof several interconnections adversely affects process control.

To reduce these limitations, a process variable transmitter provides acontrol output directly, thereby bypassing the controller itself.Outputs representative of other process variables are communicated tothe transmitter rather than the controller. The transmitter can stillcommunicate with the controller over a common two-wire link, but thelink can be broken without interrupting control. Process controlreliability and response time is enhanced and control is realized withfewer communication exchanges while installation complexity, maintenancecomplexity and cost decreases.

SUMMARY OF THE INVENTION

The present invention relates to a smart field-mounted control unitproviding a command output for controlling a process and communicatingover a two-wire circuit which powers the unit. The control unit includesan input section which receives process signals representative of morethan one process variable on the two wire circuit, storing values of theprocess signals as appropriate. The unit also includes a controllingsection coupled to the input section for providing the command outputand storing its value. The command output is a function of the storedprocess signals. A stored previous value of the command output may alsobe used in providing the command output.

One embodiment of the invention includes an energy conversion sectioncoupled to the controlling section for receiving pneumatic, hydraulic orelectromagnetic energy and for regulating such energy applied to theprocess as a function of the command output.

Another embodiment has a sensing device, coupled to the controllingsection, which senses and scales a first process variable. In such case,the process signals for determining the command output include thescaled first process variable.

In these embodiments, the process signals received from the two wirecircuit can determine operation of the controlling means in producing acommand output. Alternatively, the process signals comprise a processvariable reported to the control unit over the two wire circuit or a setof instructions sent over the two wire circuit for determining thecommand output. When the process signals comprise a setpointrepresentative of a desired state of the process, the controllingsection can generate the command output as a function of the differencebetween the setpoint and the process variable. As appropriate for theprocess to be controlled, the controlling section uses an equationincluding a linear combination of the process variable and the timeintegral of the process variable to determine the command output. As theapplication requires, the equation for determining the command outputincludes the time-rate-of-change of the process variable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of a part of a process control system having acontrol unit according to the present invention, a supervisory masterunit and a master controller;

FIG. 2 shows a block diagram of a first preferred embodiment of acontrol unit and a remote device;

FIG. 3 shows a block diagram of a second preferred embodiment of acontrol unit;

FIG. 4 shows a block diagram of a third preferred embodiment of acontrol unit coupled to a remote device; and

FIG. 5 shows a block diagram of a fourth preferred embodiment of acontrol unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an industrial process control application as in apetrochemical tank farm 1, where a fluid 2 flows in a pipe network 4. Amaster controller 6 commands a supervisory master unit 10 via a cabling12. Supervisory master unit 10 communicates over a two wire circuit 18with a feedback loop 14, which controls flow in pipe segment 20.Feedback loop 14 comprises a control unit 22 and a two terminal remotedevice 26, which controls fluid flow, Q, from a tank 30 into pipenetwork 4. The capacity of tank farm 1 can be expanded by additionalfeedback loops, located at pipe segments 20A, 20B, 20C and 20D andcontrolled by master supervisor 10. An even larger expansion requireseven more additional feedback loops and additional supervisory masterunits. Regardless of capacity, tank farm 1 is divided into a field area34 requiring intrinsically safe equipment and a control room area,indicated by block 36. An intrinsically safe barrier device 16, whichlimits voltage and current to specified levels, is mounted on cable 12at the interface between control room 36 and field 34. Each additionalcable between control room 36 and field 34 requires installation of sucha barrier device.

The flow, Q, in pipe segment 20 is given as:

    Q=k{ρ*DP}.sup.0.5                                      (1)

where Q is the mass flow rate, ρ is the density of fluid 2, DP is thedifferential pressure across an orifice in pipe segment 20 and k is aconstant of proportionality. This calculation of flow requires oneprocess variable representative of differential pressure.

However, when ρ varies as is typical in petrochemicals, a more accurateassessment of the flow, Q, is given as:

    Q=k'{AP*DP/AT}.sup.0.5                                     (2)

where AP is the absolute pressure in pipe segment 20, k' is anotherconstant of proportionality and AT is the absolute temperature of fluid2. Two additional process variables, absolute temperature and absolutepressure are required.

FIG. 2 shows a first preferred embodiment of "smart" control unit 22,communicating with supervisory master unit 10 over two wire circuit 18and comprising input means 50 and controlling means 52. "Smart" meansthat a computing capability is in the control unit, such as is performedby a microprocessor. Control units are connected in two wire circuits ina variety of ways. Each two wire circuit is coupled to a power sourcepowering instruments on the circuit. In a first configuration, thesupervisory master unit includes the power source which powers a controlunit. Additional control units may be powered by the supervisory masterunit and are connected in parallel across the power source. In a secondconfiguration, the supervisory master unit includes the power sourcewhich powers at least one control unit and at least one remote device,connected in series across the power source. In a third configuration, acontrol unit used in the first configuration powers one or more remotedevices, control units or both. The number of instruments receivingpower is typically limited by available current, but alternative powersources are sometimes available for remote devices. The remote devicemay be connected to the control unit by a pair of wires, oralternatively, by a second two wire circuit.

Control unit 22 is connected in a two wire circuit 18 as in the firstconfiguration described above, but may be connected as described abovein other configurations. The input means 50 has receiving means 54coupled to two wire circuit 18 for receiving process signals and storingmeans 56 coupled to receiving means 54 for storing process signals 55.Controlling means 52 receives process signals 55 from receiving means 54and storing means 56 of input means 50, as desired. A storage means 53receives command output 58 and outputs a previous command output 60.Command output 58 is provided by block 52a as a function of processsignals 55 and previous command output 60 and coupled to cable 57, whichmay be part of a second two wire circuit depending on the configuration.The command output 58 can be provided in some applications as a functionof process signals 55 alone. A control unit such as this is used in afeedforward control application, where no feedback is used to generatethe command output. In either application, remote device 26 is atransducer such as one which uses a current magnitude to regulate apressure and is known in the process control industry as a current topressure (I/P) converter. In a cascaded control application, however,the command output of one control unit is used as a process signalrepresentative of a setpoint for another control unit. Accordingly, in acascaded control application, remote device 26 is another control unit22.

Remote device 26 applies an energy source 59 of pneumatic air to theprocess as a function of the command output 58. Process signals used ingenerating command output 58 comprise setpoints representative of adesired process state, process variables produced by the process,commands directing the operation of controlling means 52, instructionsets in part or in whole for operation of controlling means 52,coefficients of terms for controlling means 52 and requests for statusinformation about control unit 22 from supervisory master unit 10.Different types of process signals are sent to control unit 22 dependingon control unit 22 and the process control application.

A first type of process signal is the process variable. Processvariables are categorized as primary process variables when they aredirectly representative of the variable to be controlled by the feedbackloop. The primary process variable for control unit 22 is flow.Secondary process variables affect the primary process variable and aretypically used to more accurately assess the primary process variable.Such compensation techniques are disclosed in U.S. Pat. No. 4,598,381titled Pressure Compensated Differential Pressure Sensor and Method,owned by the same assignee as this application and incorporated hereinby reference. Alternatively, two process variables representative of thesame measurand are sent to control unit 22 from different instruments onthe same feedback loop, providing redundancy of a critical measurement.In this embodiment, a transmitter 64 senses differential pressure acrossan orifice 66 in pipe segment 20 and absolute pressure via a pressureinlet 65. A temperature transmitter 68 senses absolute temperature offluid 2 via a thermocouple 69. Controlling means 52 use process signals55 representative of differential pressure, absolute pressure andabsolute temperature, sent over two wire circuit 18 from transmitters 64and 68, to adjust command output 58 according to Eq. 2.

A second type of process signal is the setpoint, indicative of a desiredprocess state. In this application an example of a setpoint is a desiredflow of 10 liters³ /minute in pipe segment 20. Typically, processsignals 55 representative of a setpoint and representative of processvariables are sent to control unit 22. The primary process variable istypically compensated by other process variables sent over circuit 18.Controlling means 52 evaluate the difference between the setpoint andthe compensated process variable and adjust command output 58.

Another type of process signal 55 is representative of commands whichselect between instructions sets stored in control unit 22 for providingcommand output 58. For example, a shut-down command causes commandoutput 58 to be governed by an instruction set for safely shutting downfeedback loop 14. Shut-down commands allow manual intervention offeedback loop 14. Another type of command instructs control unit 22 toadaptively set its own instruction set for operation of controllingmeans 52. In adaptive control, the instruction set may vary as afunction of time. In such mode, control unit 22 calculates its own termsand coefficients, as desired, for operation of controlling means 52.Alternatively, another command may cause control unit 22 to operate inan exceptional mode, where it communicates with supervisory master unit10 only if process variables exceed specified limits. Exceptional modeoperation reduces communications between instruments in tank farm 1 andreleases supervisory master unit 10 from continually communicating withfeedback loop 14. The resulting decrease in communication overhead freessupervisory master unit 10 to monitor larger numbers of feedback loopsand to perform more duties than before.

Process signals representative of instructions sets, in part or whole,are sent to control unit 22 to direct operation of controlling means 52.Partial instruction sets are sent to control unit 22 when a subset of aninstruction set is needed to adjust command output 58.

A typical instruction set adjusts command output 58. A general equationfor control is given: ##EQU1## where q_(K) is the command output at theKth time, r_(i) and y_(i) are the setpoint and process variable valuesat the ith time respectively, i varies from 0 to K, and a_(i) and b_(i)are application specific constants which may be time varying. Manyapplications require control action in which the output is proportionalto a substantially linear combination of the input process variable, thetime integral of the process variable and the time-rate-of-change of theprocess variable, sometimes called proportional-integral-derivative(PID) action. The following equations define constants a_(i) and b_(i)from Eq. 3 appropriately for PID control action. The constant a_(i) forthe present time and two previous evaluations are defined in Eq. 3.1a-c:

    a.sub.K =K.sub.P +K.sub.I.                                 (3.1a)

where K_(P) and K_(I) are defined as proportional or integral gainconstants.

    a.sub.K-1 =-K.sub.P, Eq.                                    (3.1b)

    a.sub.K-x =0, where x≧2                             (3.1c)

The constant b_(i) for the present time and two previous evaluations isdefined in Eq. 3.2a-c:

    b.sub.K ={R/(1-Q)-K.sub.P -K.sub.I }                       (3.2a)

where R=(K_(D) T_(D) N)/(T_(D) +N h), Q=T_(D) /(T_(D) +N h) , and K_(D)is a derivative gain constant, T_(D) is a derivative time constant, N isa rate limiting constant and h is a measure of the amount of timerequired to adjust command output 58. Furthermore,

    b.sub.K-1 ={-2R/(1-Q)+K.sub.P                              (3.2b)

    b.sub.K-2 =R/(1-Q)                                         (3.2c)

and all b_(K-x) =0, where x≧3.

Substituting Eq. 3.1a-c and Eq. 3.2a-c into general control Eq. 3 yieldsa three term PID control Eq. 3.3, which becomes a PI equation when thethird term is set to 0:

    Δq.sub.K =ΔP.sub.K +ΔI.sub.K +ΔD.sub.K(3.3)

where ΔP_(K) =K_(P) {E_(K) -E_(K-1) }, ΔI_(K) =K_(I) E_(K) and ΔD_(K)={R/(1-Q)}{y_(K) -2y_(K-1) +y_(K-2) } and

E_(K) =r_(K) -y_(K) is the difference between the process variable andthe setpoint at time K and represents the error.

The process control application dictates appropriateness of a PI or PIDcontrol equation. When proportional gain of an control application isrelatively low, varies over a wide span and the controlled variable isslow to change, as is typical in flow and liquid pressure applications,integral control action is necessary, while derivative control action isnot. Such control best uses a PI control action. PID control action, onthe other hand, is best suited for applications such as temperaturecontrol, where proportional gain is low, is confined to a narrow band ofvalues and the controlled variable is slow to change.

Another type of process signal is representative of coefficients forterms of an instruction set already stored in control unit 22. Forexample, if modifications in pipe network 4 were required, supervisorymaster unit 10 sends a new value of K_(P), in Eq. 3.1a, to control unit22.

A final type of process signal is representative of a request forinformation from control unit 22. This request originates from othercontrol units and from supervisory master unit 10, as desired. Statusinformation such as process control statistics, current modes ofoperation, process variable values and unit serial numbers may bemonitored.

Various types of remote device 26 can be used with control unit 22. Asdiscussed, remote device 26 is an I/P converter receiving command output58 applying pneumatic air 59 for positioning valve 62 as a function ofcommand output 58. Other process control applications may controlabsolute pressure, temperature, conductivity, pH, oxygen concentration,chlorine concentration, density, force, turbidity, motion and others. Inthese applications, remote device 26 may comprise a motor, a valve for agauge pressure application, a switch and contact as desired in atemperature controlling application, a relay in a pH or levelapplication or other implementation device.

A second preferred embodiment of control unit 22 is shown in FIG. 3.Control unit 22 as described in FIG. 2, further comprises energyconversion means 80, coupled to controlling means 52 by command output58 and controlling the process. Such an embodiment of control unit 22 isalso called a field actuator. Control unit 22 is connected in a two wirecircuit as in the first configuration described above, but can beconnected as described in other configurations. Energy conversion means80 receives pressurized air 82, commonly available in field 34 throughenergy conduit 84 and applies such air 82 to position a valve 90 as afunction of command output 58. Cabling 57 and remote device 26 (FIG. 2)are not included in this embodiment, and the process is controlled fromcontrol unit 22, instead of the remote device 26. Depending on theprocess control application, energy conversion means 80 uses other typesof energy such as hydraulic or electromagnetic energy. The frequencyrange of electromagnetic energy may range from direct current havingsubstantially zero frequency to light energy at varying frequenciescarried on an optical fiber.

A third embodiment of control unit 22 is shown in FIG. 4. The preferredfirst embodiment of control unit 22 further comprises a sensing device100 having a sensed output 112 from either a sensing means 102 or ascaling means 104. In this application, flow is given by Eq. 1,requiring only a process variable representative of differentialpressure. In sensing device 100, sensing means 102 senses pressure oneither side of an orifice 106 protruding into fluid 2. As required,scaling means 104 scales sensed process variables according to userdefinable constants such as span, zero and damping. Span and zeroadjustments allow known pressures to be referenced to the range extremesof sensing means 102 while damping affects the time response of the unitto a process variable input change. Scaling the process variablelinearly re-ranges the process variable between two user selectableoutputs typically known as "zero" and "span". A "zero" setting is anoffset adjustment in that it affects all points of the process variableequally. It indicates that a selected sensor reading is the process zeroand should result in a 4 mA loop current. A "span" switch sets theprocess maximum to 20 mA or full scale value.

The sensed output 112 of sensing means 102 and scaling means 104, asdesired, is used in controlling means 52 or coupled directly to cabling57, as desired. When sensed output 112 is coupled to cabling 57, thesignal on cabling 57 is representative of the sensed process variable,as from a transmitter. Transmitters sense process variables and output asignal representative of the sensed process variable. Transmitters areknown and disclosed in U.S. Pat. No. 4,833,922 by Frick et al. titledModular Transmitter, owned by the same assignee and incorporated hereinby reference.

Control unit 22 as shown in FIG. 4 is connected in two wire circuit 18per the second configuration as discussed above, but may be connected inother configurations. One terminal of remote device 26 is connected viacable 57 to control unit 22 while the other is connected to mastersupervisor 10 by two wire circuit 18.

The ability of control unit 22 to function as a transmitter or acontroller allows use of several types of process signals. Differentoptions are available for process signals representative of processvariables.

Process signals 55 representative of a process variable and thoserepresentative of a setpoint are sent over two wire circuit 18 and usedby controlling means 52 for providing command output 58. Processvariables sent to control unit 22 are typically representative ofsecondary process variables as when Eq. 2 is used to calculate flow andtypically compensate the primary process variable sensed by sensingdevice 100. Alternatively, process signals 55 representative of aredundantly sensed primary process variable are sent via two wirecircuit 18 for increased reliability in critical applications. A thirdcommand provides for simultaneous execution of the controlling mode andtransmitter mode. In such dual mode operation, command output 58 iscoupled to cable 57 in an analog fashion and the magnitude of cable 57current varies as command output 58. Remote device 26 adjusts valve 62as a function of cable 57 current magnitude. Several communicationstandards vary current magnitude as a function of the information sent,such as 4-20 mA and 10-50 mA current loop communications standards.Alternatively, the voltage on cable 57 is representative in a voltagemagnitude communication standard such as 1-5V. Concurrently, controlunit 22 digitally couples sensed process variable 112 to cable 57 in acarrier modulated fashion. For example, signals representative of acommand output are encoded by the 4-20 mA standard and signalsrepresentative of a process variable are digitally encoded by a carriermodulated format. Typical carrier modulation communication standardswhich may be used are frequency shift key (FSK), amplitude modulation(AM), phase modulation (PM), frequency modulation (FM), QuadratureAmplitude Modulation (QAM) and Quadrature Phase Shift Key (QPSK).Alternatively, a baseband communication standard such as Manchester isused to encode process variable 112 on two wire cable 57.

Master supervisor 10 monitors process variables while control unit 22controls the remote device 26 simultaneously, due to the seriesconnection of master supervisor 10, control unit 22 and remote device 26and because remote device 26 cannot change current on circuit 18 due toits passive nature. Such operation provides cost and efficiencyadvantages, because the number of two wire circuits needed for processcontrol is reduced from two circuits to one circuit for each feedbackloop. Absent such mode, a first two wire circuit communicates a processvariable between a transmitter and master supervisor 10 and a second twowire circuit communicates a command output between master supervisor 10and remote device 26. With such mode, a single two wire circuitconnecting master supervisor 10, control unit 22 and remote device 26 inseries controls the process. Wiring costs in field areas are expensive,with each feedback loop wiring representing approximately the sameinvestment as a transmitter and a remote device. Secondly, processsignals 55 representative of commands sent over two wire circuit 18select between the two operational modes. This command directs controlunit 22 to couple sensed process variable 112 or command output 58 ontocabling 57. Upon such command, the same control unit 22 functions as atransmitter or a controller, respectively. During operation as acontroller, a command directing operation on exceptional basis is sentover circuit 18. Exceptional basis operation instructs control unit 22to communicate with master supervisor 10 only when process variablesreceived or sensed by the unit are not within specific limits. Suchcommands obviate master supervisory 10 monitoring or intervention incontrol unit 22 operation, resulting in fewer communications to maintainprocess control. Another advantage is increased reliability of processcontrol, since cable 12 may be broken without compromising the processin this mode.

Thirdly, process signals 55 representative of varied instruction setsare sent to control unit 22 as appropriate for the process controlapplication discussed above. Diverse functionality in variedapplications is achieved. For example, control unit 22 sensesdifferential pressure in a process controlling flow when a firstinstruction set governs controlling means 52 and senses differentialpressure in a process controlling level when a second instruction setgoverns controlling means 52. Alternatively, control unit 22 providescommand output 58 to different types of remote devices 26, changing suchcommand output 58 as a function of process signals 55.

A fourth preferred embodiment of control unit 22 is shown in FIG. 5.Energy conversion means 80, as shown in FIG. 3, is coupled to controlunit 22 as described in the third preferred embodiment of control unit22, and functions as discussed there. Cabling 57 and remote device 26are eliminated in this embodiment, and the process is controlled fromcontrol unit 22. Energy conversion means 80 uses other types of energysuch as hydraulic or electromagnetic energy.

What is claimed is:
 1. A smart field-mounted control unit for couplingto a two-wire process control loop comprising:a single enclosure; inputmeans carried in the single enclosure coupled to the two-wire processcontrol loop for receiving power over the two-wire process control loop,the input means comprising: receiving means for receiving processsignals representative of more than one process variable sent by amaster unit over the two-wire process control loop, and for sendinginformation over the two-wire process control loop to the master unit;means for storing process signals received from the master unit whichare representative of more than one process variable; and controllingmeans carried in the single enclosure coupled to the input means forproviding a command output for controlling a process and for storingprevious values of the command output where the command output is afunction of the stored process signals and a stored previous value ofthe command output.
 2. The apparatus as cited in claim 1 where theprocess signals further comprise means for determining the function forproviding the command output.
 3. The apparatus as recited in claim 1where the process signals further comprise instructions for operation ofthe controlling means.
 4. The apparatus as recited in claim 1 where theprocess signals further comprise commands directing operation of thecontrolling means.
 5. The apparatus as recited in claim 1 where theprocess signals further comprise setpoints and where the controllingmeans adjust the command output as a function of the difference betweenthe setpoint and one of the process variables.
 6. The apparatus asrecited in claim 1 where the process signals are digitally representedon the two-wire process control loop.
 7. The apparatus as recited inclaim 1 where the command output is coupled in an analog manner to aremote device over the two-wire process control loop.
 8. The apparatusas recited in claim 1 further comprising:a sensing device coupled to thecontrolling means and having means for sensing and scaling a firstprocess variable; and where the process signals for determining thecommand output further comprise the scaled first process variable.
 9. Asmart field-mounted control unit for controlling a process,communicating over a two-wire process control loop which powers theunit, comprising:a single enclosure; input means carried in the singleenclosure coupled to the two-wire process control loop for receivingpower over the two-wire process control loop, for receiving processsignals over the two-wire process control loop from a master unitrepresentative of more than one process variable, for sendinginformation to the master unit over the two-wire process control loop,and for storing process signals; controlling means carried in the singleenclosure coupled to the input means for providing a command output forcontrolling the process and for storing previous values of the commandoutput, where the command output is a function of the stored processsignals and a stored previous value of the command output; a source ofenergy capable of being regulated for controlling the process; andenergy conversion means carried in the single enclosure coupled to thecontrolling means for regulating energy from the source and for applyingthe energy to the process as a function of the command output.
 10. Theapparatus as recited in claim 9 where the process signals furthercomprise means for determining the function for providing the commandoutput.
 11. The apparatus as recited in claim 9 where the processsignals further comprise instructions for operation of the controllingmeans.
 12. The apparatus as recited in claim 9 where the process signalsfurther comprise commands for directing operation of the controllingmeans.
 13. The apparatus as recited in claim 9 where the process signalsfurther comprise setpoints and where the controlling means adjust thecommand output as a function of the difference between the setpoint andone of the process variables.
 14. The apparatus as recited in claim 9where the process signals are digitally represented on the two-wireprocess control loop.
 15. The apparatus as recited in claim 9 where thecommand output is coupled in an analog manner to a remote device overthe two-wire circuit.
 16. A smart field-mounted two-wire transmitter,comprising:a single enclosure; a sensing device carried in the singleenclosure having means for sensing and scaling a first process variable;input means carried in the single enclosure coupled to a two-wireprocess control loop for receiving power over the two-wire processcontrol loop, receiving process signals over the two-wire processcontrol loop from a master unit, and for storing process signals; andcontrolling means carried in the single enclosure coupled to the sensingdevice and input means, for providing a command output to a remotedevice which controls a process and for storing the command output,where the command output is a function of the stored process signals,the scaled first process variable and a previously stored value of thecommand output.
 17. The apparatus as recited in claim 16 where theprocess signals further comprise means for determining the function forproviding the command output.
 18. The apparatus as recited in claim 16where the process signals further comprise instructions for operation ofthe controlling means.
 19. The apparatus as recited in claim 16 wherethe process signals further comprise setpoints and where the controllingmeans adjusts the command output as a function of the difference betweenthe setpoint and the scaled process variable.
 20. The apparatus asrecited in claim 16 where the controlling means adjusts the commandoutput proportionally to a substantially linear combination of theprocess variable, the time integral of the process variable and thetime-rate-of-change of process variable.
 21. The apparatus as recited inclaim 20 where the controlling means adjusts the command output, q_(k),according to the equation, ##EQU2## where q_(k) is the command output atthe kth time, r_(i) and y_(i) are the setpoint and process variablevalues at the ith time respectively, i varies from 0 to k and a_(i) andb_(i) are application specific constants which may be time varying. 22.The apparatus as recited in claim 16 where the process signals aredigitally represented on the two-wire process control loop.
 23. Theapparatus as recited in claim 22 where the process signal coupling isselected from the group of communication standards comprising carriermodulation and baseband.
 24. The apparatus as recited in claim 16 wherethe command output is coupled in a analog manner to the remote deviceover a pair of wires.
 25. The apparatus as recited in claim 24 where theformat for analogly coupling the command output to the pair of wires isselected from the group of formats comprising 4-20 mA, 10-50 mA and1-5V.
 26. The apparatus as recited in claim 16 where the sensing devicefurther comprise sensors selected from the group of sensors sensingpressure, temperature, flow, mass, conductivity, moisture, pH, oxygenconcentration, chlorine concentration, density, force and turbidity. 27.An apparatus for improving a smart transmitter which is powered by atwo-wire process control loop and which is coupled to and communicateswith a remote device which controls a process, the transmitter havingsensing means for sensing a first process variable, scaling meanscoupled to the sensing means for scaling the sensed process variable,communications means coupled to the scaling means for coupling thesensed process variable to the two-wire process control loop,characterized in that the apparatus comprises:a single enclosure; inputmeans carried in the single enclosure for input and storage coupled tothe two-wire process control loop for receiving process signals from amaster unit over the two-wire process control loop, and for receivingpower over the two-wire process control loop; and controlling meanscarried in the single enclosure coupled to the input means for providinga command output to the remote device and storing the command output,where the command output is a function of the process signals, thescaled process variable and a previously stored value of the commandoutput.
 28. The apparatus as recited in claim 27 where the processsignals further comprise means for determining the function forproviding the command output.
 29. The apparatus as recited in claim 27where the process signals further comprise instructions for operation ofthe controlling means.
 30. The apparatus as recited in claim 27 wherethe process signals further comprise setpoints and where the controllingmeans adjust the command output as a function of the difference betweena setpoint and the scaled process variable.
 31. The apparatus as recitedin claim 27 where the controlling means adjusts the command outputproportionally to a substantially linear combination of the processvariable, the time integral of the process variable and thetime-rate-of-change of process variable.
 32. The apparatus as recited inclaim 31 where the controlling means adjusts the command output, q_(k),according to the equation, ##EQU3## where q_(k) is the command output atthe kth time, r_(i) and y_(i) are the setpoint and process variablevalues at the ith time respectively, i varies from 0 to k and a_(i) andb_(i) are application specific constants which may be time varying. 33.The apparatus as recited in claim 27 where the process signals aredigitally represented on the two-wire process control loop.
 34. Theapparatus as recited in claim 27 where the command output is analoglycoupled to the remote device over a pair of wires.
 35. The apparatus asrecited in claim 27 where the sensing means further comprise sensorsselected from the group of sensors sensing pressure, temperature, flow,mass, conductivity, moisture, pH, oxygen concentration, chlorineconcentration, density, force and turbidity.
 36. The apparatus asrecited in claim 27 where the process signals are representative ofredundant process variables.
 37. A smart field-mounted transmitterpowered by a first two-wire circuit and sending an output onto a secondtwo-wire circuit, comprising:a single enclosure; a sensing devicecarried in the single enclosure having means for sensing and scaling afirst process variable; input means carried in the single enclosurecoupled to the first two-wire circuit for receiving other processvariables from a master unit over the first two-wire circuit, fortransmitting information to the master unit over the two-wire circuit,and storing the other process variables; compensation means carried inthe single enclosure coupled to the input means for compensating one ofthe process variables as a function of the other process variables; andoutput means carried in the single enclosure coupled to the compensationmeans for coupling the compensated process variable to the secondtwo-wire circuit.